As any school child will tell you, the largest known animal now and in evolutionary history is the blue whale. At 30 metres long and weighing up to 190 tonnes, blue whales may be about as big as it is possible for this type of animal to get. Any larger and they would face overheating on exertion, with their metabolism generating heat faster than it could be dissipated through their surface area. The problem is compounded by their thick layer of blubber, essential for insulation when at rest.
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